Modular conveyor belts have been highly developed over the years. The belt links making up the conveyor typically each have a plurality of spaced link ends which intermesh with complimentary link ends projecting from a link or links in an adjacent row. A connecting pin runs through the meshing link ends to permit pivotal movement of the adjacent links such that the adjacent links are connected in a hinge-type configuration. Typically, the connecting pins include ends at each side edge of the conveyer belt, and the ends have a head that prevents the connecting pin from moving transversely relative to the conveyor. The head on one end of the connecting pin can be preformed, but the heads may also be formed by heat and pressure after the pin has been placed in position. Alternatively, plugs can be inserted into the openings in the link ends at each side edge of the conveyor belt. These plugs restrain the pins within the link ends.
The individual links which comprise modular conveyor belt are typically similar in width and are arranged in a bricked configuration. During operation the links may be subject to forces that tend to separate laterally adjacent links. The connecting pins are subjected to localized sheer stresses due to the tensile loading of the chain links.
The connecting pins are also prone to bending between the link ends of the conveyor modules. This pin bending causes point contact between the connecting pins and the internal edges of the link ends. Where there is point contact between the link ends and the connecting pin (instead of preferred surface contact between the pin and the inner bearing surface of the link ends), the connecting pin and the link ends tend to wear significantly.
Conventional connecting pins are typically made of thermoplastic (e.g., acetal, polyester, nylon and polypropylene). The choice of the thermoplastic used for the connecting pin usually depends on the physical properties which are desired (i.e., high-strength, low friction and/or suitability for use under extreme cyclic temperatures, chemical resistivity). The strength of the pin is especially important because a stronger pin with increased mechanical properties such as shear strength, tensile strength, tensile modulus of elasticity, flexural modulus of elasticity increases the mechanical properties of the modular conveyor belt and reduces stretch due to loading.
Another important consideration in selecting the material for a connecting pin is the need to control the expansion of the modular conveyor belt under extreme cyclic temperatures and high tensile loading. One typical application where a modular conveyor belt is subject to extreme cyclic temperature and high tensile loading is where cans or bottles are transported through pasteurizers used in breweries. The high temperatures in a pasteurizer and the slow movement of the cans or bottles through the pasteurizer cause the chain to stretch such that the bottom portion of an endless conveyor chain sags. The chain also stretches considerably due to tensile load. This chain stretching effects the performance of the drive sprocket and conveyor link interaction. In addition, in double deck conveyor systems, the sagging can become so great that the bottom portion of an endless conveyor belt can knock over bottles located on the top of a lower conveyor belt.
As stated previously, the connecting pins and link ends wear during operation of the conveyor. This wear is caused by abrasion between the connector pins and link ends. Some of the conventional connector pins are made from low-friction thermoplastics characteristics that minimize wear between the connector pins and the link ends. However, low friction thermoplastics typically lack other characteristics which are desired in a connector pin.
One typical connector pin is disclosed in U.S. Pat. No. 4,240,605. This patent discloses a flexible hinge pin for connecting belt fasteners. The pin has multiple metallic wire strands which make up the central stranded core. The strands are then wrapped with a metallic ribbon and the assembly is covered with a flexible Nylon covering. The disclosed pin is intended to provide high flexibility.
Another connector pin is disclosed in U.S. Pat. No. 4,230,239. This patent discloses a flexible coupling rod for connecting the ends of a conveyor belt. The coupling rod comprises a flexible core and pushed-on interlocking metallic sleeves.
Another known connector pin is disclosed in U.S. Pat. No. 5,461,852. This patent discloses a chain and connecting pin made from fiber reinforced thermoplastic resin. The pin is formed by injection molding glass fiber reinforced polyamide.
Another such connecting pin is disclosed in U.S. Pat. No. 5,125,874. The disclosed connector pin comprises rectangular hinge strips made of a flexible material for use with a modular conveyor belt suitable for abrasive environments. The hinge strips are typically made from a thermoplastic resin which is extruded to properly orient the grain structure.
A connecting pin is also disclosed in U.S. Pat. No. 4,903,824. The patent discloses a carbon or ceramic conveyor belt hinged with connecting pins made from the same carbon or ceramic material. The connecting pins are made by using the carbon or ceramic fibers twisted to and stiffened by carbonic adhesive to linear wires.